Interesting ...
When I create a new application, place some standard controls on it like a TButton, TCheckbox, TRadioButton, Tedit, TLabel, run it, and then toggle dark/light mode. This just works out of the box for all these controls.
I'm running Lazarus 2.1.0 r59760 FPC 3.0.4 x86_64-darwin-cocoa (alpha), on MacOS Mojave 10.4.1.
In the past I tried it on High Sierra as well, which worked even there (but no longer have a Mac running High Sierra).
Do you have an example project?
p.s. When creating a new project, I just have it create the out-of-the-box bundle (.app), no special settings in the plist file.